1. Field of the Invention
This invention is in the field of syringes and more particularly relates to syringes adapted for extruding heated thermoplastic materials through a hollow needle.
2. Description of the Prior Art
Presently, root canal cavities as well as other dental cavities are obturated by filling them with heated gutta-percha. Gutta-percha is a purified, coagulated, milkly exudate obtained from certain species of trees, and is defined as a trans isomer of rubber. It becomes plastic at about 140.degree. F., and will flow through hollow needles of the type used in dental syringes at around 230.degree. F.
The method most commonly employed for filling dental cavities with gutta-percha is to apply small amounts of heated gutta-percha into the cavity with a hot probe and to compact the material within the cavity using a condensor. This is a very time consuming and tedious process, at best, however, and may, in some cases, not result in complete filling and sealing of the dental cavity.
Recently, it has been recognized that the application of injection molding principles to obturate root canal cavities with gutta-percha offers great promise. It has been demonstrated, for example, by researchers in the field of endodontics that gutta-percha can be used to obturate root canal cavities using injection molding principles. See Yee, F. S., Marlin J., Krakow, A. P., and Gron, P.; "Three-Dimensional Obturation of the Root Canal Using Injection-Molded, Thermoplasticized Dental Gutta-Percha"; J. Endodontics, 3; no. 5; May, 1977; pp 168-174. These experiments were performed in vitro, but their application in vivo would seem possible if suitable equipment were available. In addition to achieving outstanding sealing of the cavities, the techniques seem to offer potential advantages in their simplicity, reduction in time by skilled personnel, and reduction in the amount of manipulative procedures necessary for condensation of gutta-percha. Despite such promise, these techniques have not been widely accepted due to the lack of suitable equipment, such as dental syringes, for carrying out such techniques.
There have been hypodermic needles and tools which have heated tips, of course. For example, Piper et al., in U.S. Pat. No. 3,698,394, disclose a surgical hypodermic needle wherein the tip of the needle is heated with an electric current passed through resistance wires within the needle adjacent to its tip. This needle is used to destroy blood vessels or tissues. Clearly, such a needle would be unsuitable for filling dental cavities, such as those formed in root canal work, since the heating wires and insulation surrounding the needle make it so bulky that it would be difficult to insert and manipulate into a root canal cavity.
Malisza, in U.S. Pat. No. 3,614,389, discloses an electrically heated temperature control waxing apparatus for use in making wax models for dental purposes. However, this apparatus does not include an elongated hollow needle of the type necessary to insert into root canal cavities. In fact, the problem of supplying sufficient heat to a small diameter hollow needle so that its entire length, including its tip, stays above the temperature at which the material being extruded is plastic is very difficult in view of the convection losses which occur along the needle. In applications such as obturation of root canal cavities, the needle requires a minimum length, often one inch or more, and it is difficult to maintain the required temperature along the entire length of such needles.
Thus, despite great promise, injection molding principles have not been widely accepted by the dental profession for obturating root canal cavities. This is principally due to the lack of adequate equipment for applying such principles to this problem.